Conventionally, thermal transfer method has been widely used as a simple printing method. The melt-transfer method as one of the thermal transfer method is an image forming method wherein a thermal transfer sheet, which has a thermally fusible ink layer comprising a colorant, such as a pigment, and a binder such as a thermally fusible wax or resin, is superposed on a thermal transfer image-receiving sheet such as a paper or a plastic film; then, an energy in accordance with image information is applied from the back side of the thermal transfer sheet by a heating means such as a thermal head; and thereby the colorant is transferred onto the thermal transfer image-receiving sheet together with the binder. The image formed by the melt-transfer method is suitable for recording binary images such as characters, since the image has excellent sharpness and high concentration.
On the other hand, the sublimation transfer method as another one of the thermal transfer method is another image forming method wherein a thermal transfer sheet, which has a colorant layer comprising a dye which is able to be transferred thermally by sublimation, is superposed on a thermal transfer image-receiving sheet in which a dye receiving layer is provided on a substrate, then, an energy in accordance with image information is applied from the back side of the thermal transfer sheet by a heating means such as a thermal head; and thereby only the sublimable dye is transferred onto the thermal transfer image-receiving sheet. With respect to the sublimation transfer method, since the transferring amount of dye can be controlled by the amount of energy applied, it is possible to form a gray-scale image in which image density is controlled. In addition, since the colorants used are dyes, the image formed has transparency. Thus, when dyes of different colors are superposed, the reproducibility of neutral tints becomes excellent. Therefore, when using thermal transfer sheets of different colors such as yellow, magenta, cyan, black or the like, and transferring dye of each color on the thermal transfer image-receiving sheet so as to superpose the dyes each other, it is possible to form a photographic full-color image of high quality in which reproducibility of neutral tints is excellent. Furthermore, since the sublimation transfer method can express the full color image as the original onto the thermal transfer image-receiving sheet clearly, it is applied to a color image forming for a digital camera, a video camera, a computer or the like. The image is one that is high-quality comparable to silver halide photography.
In general, the thermal transfer sheet is stored and used in wound state. When the colorant contained in the colorant layer is present in a state of being localized to the surface of the colorant layer by bleeding, etc., the sublimable dye tends to migrate to the back face layer side of the thermal transfer sheet (so-called “kick”). When the sublimable dye migrated to the back layer side is again re-migrated to the colorant layer side (so-called “back”), in particular, in the case that, in the thermal transfer sheet in which a plural numbers of colorant layers which are different in the hue from each other are provided so as to be layered in parallel on the substrate sheet across the surface of the substrate sheet, as being frame sequentially, the sublimable dye migrated to the back layer side is again re-migrated to another colorant layer which is different in the hue from the sublimable dye, a decrease in color development property would be caused at the time of image formation using the another colorant layer.
Under these circumstances, with respect to the thermal transfer sheet capable of suppressing the occurrence of kick, various studies have been made. For example, in Patent literature 2, a thermal transfer sheet which comprises a dye layer provided on one surface of the substrate sheet, and a back face layer provided on another surface of the substrate sheet, wherein the dye layer contains an indoaniline based dye, a polyvinyl acetal resin A, and a polyvinyl acetal resin B (represented by the formula (1) in Patent literature 2), has been proposed. Further, in Patent literature 3, a thermal transfer sheet which comprises a dye layer provided on one surface of the substrate sheet film and containing a resin and a dye, and a back face layer provided on another surface of the substrate sheet and containing a lubricant and a resin, wherein the dye layer contains a prescribed dye (the dye represented by the formula (1) in Patent literature 3) in a prescribed amount range, has been proposed. According to the thermal transfer sheets proposed by Patent literatures 2 and 3, it is stated that it is possible to prevent the migration of colorant to the back layer side during the storage of the thermal transfer sheet.
Further, in the thermal transfer sheet proposed in Patent Literature 2, since the kind of the dye contained in the dye layer, and the kind of the binder resin are limited to the prescribed components, a problem such that the width of the material selection becomes narrower is inherent.